1. Field of the Invention
This invention relates to a process for preparing 2-amino-5-nitrophenol derivatives and more particularly, to a process for preparing 2-amino-5-nitrophenol derivatives in which an appropriate nucleophilic group is introduced at the 4 position of the derivative.
2. Description of the Prior Art
2-Amino-5-nitrophenol derivatives are generalpurpose compounds for use as industrial starting materials, and can be converted into synthetic intermediates of higher added value by the reduction of the nitro group. The compounds have the o-aminophenol structure which may serve as a reducing agent, and the degree of reducibility can be arbitrarily controlled by the introduction of a suitable substituent and the conversion of the nitro group into another functional group. In this sense, the derivatives are important as diversity of reducing agents or antioxidants or synthetic intermediates which are convertible into physiologically active compounds by the modification of the nitrogen atom.
Moreover, 2-amino-5-nitrophenol derivatives are also important as synthetic intermediates for cyan-imageforming couplers in the field of photographic chemistry. In recent years, it has been found that 2,5-diacylaminophenol cyan-couplers show good color restoration upon development and the resulting dyes have good fastness to heat and/or humidity (see, for example, Japanese Patent Application (OPI) Nos. 110530/78, 163537/80, 29235/81, 55945/81, 31953/84 and 31954/84, and U.S. Pat. Nos. 4,124,396 and 4,341,864). It has been also found that dyes produced from 2-phenylureido-5-acylaminophenol cyancouplers are excellent in color restoration upon development, absorption wavelength, and fastness to heat and/or humidity (see, for example, U.S. Pat. Nos. 4,333,999 and 4,427,767 and Japanese Patent Application (OPI) Nos. 204543/82, 204544/82 and 204545/82). Accordingly, 2-amino-5-nitrophenol derivatives have attracted attention for use as synthetic intermediates for those couplers.
Photographic couplers may be broadly classified with respect to the hue of developed dye. They may also be stoichiometrically classified into two broad classes, i.e. 2-equivalent couplers and 4-equivalent couplers. While 4-equivalent couplers require four moles of silver halide to be developed into one mole of dye, 2-equivalent couplers have a split-off group at the coupling position thereof and can form one mole of dye using two moles of silver halide. For this reason, it is known that 2-equivalent couplers are more beneficial from the standpoint of silver saving. With regard to cyan-developing couplers, for instance, 2-equivalent couplers have such a high colordeveloping speed that photographic senistivity is much improved (see, for example, U.S. Pat. Nos. 3,476,563, 3,617,291, 3,880,661, 4,052,212 and British Patent Nos. 1,531,927 and 2,006,755, and Japanese Patent Application (OPI) Nos. 32071/80, 1938/81 and 27147/81).
As recent color negative films increase in sensitivity, 2-equivalent couplers of high color developing speed, in which split-off groups are introduced at the coupling position, have been employed in large amounts. Accordingly, of increasing importance are 2-amino-5-nitrophenol derivatives and processes for preparing such derivatives.
As explained above, 2-amino-5-nitrophenol derivatives in which substituents are introduced into the benzene ring are important as industrial starting materials, reducing agents, and intermediates for preparing cyan couplers in photographic chemistry. Preparation of these derivatives is described, for example, in U.S. Pat. No. 3,880,661, and Japanese Patent Application Nos. 145333/83, 157423/83, 158470/83, 157424/83, and 199,696/83.One such example may be represented by the following reaction formula (a) ##STR1## wherein R.sub.4 and R.sub.5 each represents a hydrogen atom or a substituent, and X.sub.2 represents a nucleophilic group.
The above substitution reaction per se is known as an aromatic nucleophilic substitution reaction. This reaction is described in detail, for example, in Jerry March "Advanced Organic Chemistry" (second edition, (1977), Mcgraw-Hill Kogakusha, Ltd.), chapter 13 entitled "Aromatic Nucleophilic Substitution" pp. 584-595. In the Journal of American Chemical Society, Vol. 79, p. 385 (1957), J. F. Bunnet et al report that the reaction between 2,4-dinitrobenzene derivatives and pyridine proceeds about 3300 times more rapidly when the split-off group is a fluorine atom than when the split-off group is a chlorine or bromine atom.
However, the conventional processes of synthesis, as exemplified as formula (a), have a number of disadvantages.
(1) The fluoro derivative of the general formula [VII], used as the starting material, is obtained by five steps starting from p-fluorophenol and is thus complicated in the preparation steps.
(2) The starting p-fluorophenol is not readily available and is expensive.
(3) Because fluorine ions are generated by the reaction, additional plant investment is necessary for safety and waste water disposal.
(4) The type of material for reactors is limited.
These disadvantages place a serious limitation on mass production. On the other hand, in order to overcome the above disadvantages, it is a matter of course that the reaction is effected using, instead of fluoro derivatives of general formula [VII], chloro derivatives of the following formula [IX] as described in U.S. Pat. No. 3,880,661 ##STR2## wherein R.sub.4 and R.sub.5 each has the same meaning as defined in formula (a), respectively. These chloro derivatives are prepared from 4-chloroaminophenol which are inexpensively available in large amounts. As expected, however, the chloro derivatives are much less reactive than fluoro derivatives [VII], thereby giving low yields. Even though the reaction is carried out in the presence of a catalyst such as Cu, CuI, CuI.sub.2, CuCl.sub.2, CuBr.sub.2, CuO (Ullmann reaction, Fanta, Synthesis, 9, 12, 1974), the yield of desired product [VIII] is about 7% in a maximum with the balance of by-products with identified structures.